In a disk file, the most common recording medium is a very flat and smooth aluminum-alloy substrate disk having both of its surfaces coated with a magnetic-recording material such as a ferrimagnetic or ferromagnetic oxide powder dispersed in a resin binder or a plated or sputter-deposited thin film of ferromagnetic cobalt alloy.
Typically, an aluminum-alloy substrate of a magnetic-recording disk has surfaces that are either diamond tool turned on a lathe or ground by a surface grinder. These machined surfaces result in matte finishes. Alternatively, substrate disks may be precision blanked from flat precision cold-rolled aluminum-alloy sheet or other metal strip whose surface finish would replicate that of the work rolls used in the finishing pass of the rolling mill. For example, with work rolls that have been ground and polished to a mirror-bright finish, a metal strip with a mirror-bright surface finish would result.
Aluminum is a soft metal and, therefore, can be damaged by a read/write head impacting the disk too forcefully. To provide for protection against defects caused by impacts of a read/write head, an aluminum-alloy substrate is typically first coated with a hard, nonmagnetic material before the magnetic-recording material is applied. A nickel-phosphorus alloy, electroless-deposited from an aqueous solution, is the hard material commonly used for this application. In order for this protective plating to adhere properly to the surface of an aluminum-alloy substrate, a zincate solution is used to dissolve the surface aluminum oxides, hydroxyoxides, and hydrous oxides, and to provide a zinc metal layer by replacement reaction. After coating, the surface of the nickel-phosphorus-alloy-coated disk is lapped and polished to provide a flat and smooth surface for the application of the magnetic layer.
Apart from the need to lap and polish the disk after application of the nickel-phosphorus alloy, the application of this hard coating presents additional difficulties. For example, it is extremely difficult to obtain flaw-free electroless-deposited nickel-phosphorus-alloy coatings. Nodules, pits, and bumps occur in these coatings and such defects cause recording errors.
Another problem is that the electroless-deposited nickel-phosphorus alloy is very prone to recrystallization upon heating, where the nonmagnetic (actually, superparamagnetic) single phase of nickel and phosphorus, separates into two equilibrium crystalline phases, namely, nickel, which is ferromagnetic, and nickel phosphide. The resulting ferromagnetism renders the media useless for the magnetic-recording application.
Moreover, such a disk is also subject to warpage upon heating because of stress concentrations at the coating-substrate interface. This warpage also renders the media useless.
Another problem associated with the use of an aluminum alloy as a substrate includes the added cost for time and labor involved in its processing. Furthermore, entire disks can be rendered useless through a myriad of heat-related effects.